This invention relates to a speed control system of a synchronous motor, characterized in that it utilizes such detector as an Inductsyn sold by Inductsyn Corp., U.S.A. or a resolver which produces a phase modulated signal used as a feedback signal representing the rotation or speed and the rotational angle of the motor. In a synchronous motor provided with a permanent magnet mounted on a rotor for producing a rotating field as in a brushless DC motor, and a stator winding, the torque generated by the motor somewhat varies depending upon the rotational position of the permanent magnet so that where such synchronous motor is used as a servomotor there is a disadvantage that the generated torque varies, especially at the time of starting.